1. Field of the Invention
The present invention relates to a continuous sintering furnace and the method of use thereof. The continuous sintering furnace concerned is a sintering furnace for continuously sintering work or material to be sintered into ceramics. The works or materials to be sintered may be carbonic, nitric and oxide ceramics such as alumina (Al2O3), silicon carbide (SiO) and boron nitride (BN) and their treatment temperature maybe more than 1600xc2x0 C. Structure, furnace material and the mechanisms needed are essential factors for such sintering furnace being operated continuously at high temperature. Usually graphite is used as furnace material because of its excellence in heat resistance, which may bring about considerable restrictions in the structure and mechanisms needed because of its physicality.
2. Discussion of the Background
FIGS. 1 and 2 show a conventional continuous sintering furnace comprising an entrance-side deaerating chamber 3 through which trays 2 each with a work or material 1 to be sintered being mounted thereon may pass, a furnace body 5 which is arranged in a chamber 4 contiguous with said deaerating chamber 3 and into which the trays 2 are sequentially fed in a line or column and an exit-side deaerating chamber 6 which is contiguous with said chamber 4 and through which the trays 2 having passed through the furnace body 5 may pass.
A space between an inner face of the chamber 4 and an outer face of the furnace body 5 is filled with heat insulating material (not shown). A double-walled cooling structure is applied to the chamber 4.
The deaerating chamber 3 is provided with vertically movable doors 7 and 8 at its upstream and downstream ends in a direction of transportation of the trays 2, respectively. Likewise, the deaerating chamber 6 is provided with vertically movable doors 9 and 10 at its upstream and downstream ends in the direction of transportation of the trays 2, respectively.
With the doors 7, 8, 9 and 10 being closed into their lowered positions, air-tightness is maintained in the chambers 3, 4 and 6. With the doors 7, 8, 9 and 10 being opened into their raised positions, the trays 2 are allowed to pass through the chambers 3, 4 and 6.
In the chambers 3, 4 and 6 and along substantially the entire length thereof, pairs of laterally spaced skid beams 11, 12 and 13 are provided to slidably support the trays 2 from below, respectively.
A plurality of vertically extending heaters 14 are disposed in a longitudinally intermediate portion of the furnace body 5 such that the heaters 14 are positioned laterally of the material 1 to be sintered on the tray 2. The material 1 to be sintered is heated by the heaters 14.
The continuous sintering furnace is also equipped with a pusher 15 which pushes the trays 2 one by one into the furnace body 5 from the deaerating chamber 3 as well as a puller 16 which pulls the trays 2 one by one from the furnace body 5 to the deaerating chamber 6.
Upon starting of an operation of the continuous sintering furnace, the furnace body 5 is filled with non-oxidizing gas with the doors 8 and 9 being closed. Then, the heaters 14 are activated to heat the inside of the furnace body 5 to a predetermined temperature.
Next, the tray 2 on which the material 1 to be sintered is mounted is fed to the deaerating chamber 3; and the door 7 is closed and air inside the chamber 3 is discharged. Then, the door 8 is opened and the tray 2 is pushed into the furnace body 5 by the pusher 15; and the door 8 is closed again.
After the lapse of a predetermined time period, another tray 2 is pushed from the deaerating chamber 3 into the furnace body 5 according to the procedure described above to thereby push the tray or trays 2 already in the latter toward the deaerating chamber 6.
Repetition of the procedure described above causes the tray 2 to reach the most downstream position in the furnace body 5. Then, the door 9 is opened with the door 10 being closed; and the tray 2 is pulled by a puller 16 from the furnace body 5 into the deaerating chamber 6. After closing the door 9, the door 10 is opened to take the tray 2 to outside.
Thus, the material 1 is gradually raised in temperature for a predetermined time period in a preheating zone 17 in the furnace body 5 adjacent to the deaerating chamber 3, is heated to a constant temperature for a predetermined time period in a heating zone 18 at the intermediate portion in the furnace body 5 and is gradually cooled for a predetermined time period in a gradual cooling zone 19 in the furnace body 5 adjacent to the deaerating chamber 6.
In the continuous sintering furnace constructed as described above and when the amount of production is to be increased without changing a cross sectional area of the furnace, the heating zone 18 is prolonged in length and movement of the tray 2 is increased in speed.
When a variety of products are required to be produced for small quantities, the heating zone 18 is shortened in length and movement of the tray 2 is decreased in speed so as to reduce the number of production lots.
The continuous sintering furnace shown in FIGS. 1 and 2 may be suitable for a single product with a certain degree of large-scale production. However, in multiple products with small-scale production in which the heating zone 18 is shortened in length and movement of the tray 2 is decreased in speed, tact time of the material 1 becomes longer so that thermal loss in the heating zone 18 increases, resulting in heat input to the gradual cooling zone 19. Consequently, the gradually cooling zone 19 must be prolonged in length so as to secure sufficient cooling time for the work or material 1.
Use of different process gases in the heating zone 18 and gradually cooling zone 19 would result in mixture of the two gases since the zones 18 and 19 are always in communication with each other.
An intermediate door cannot be provided between the zones 18 and 19 for avoidance of such mixture of the two gases since the construction is such that the tray 2 pushed into the zone 17 pushes the tray or trays 2 already in the zones 17, 18 and 19 downstream in the direction of transportation.
A furnace floor structure is provided by skid beams 12; there is high sliding friction coefficient between the tray 2 and the skid beams 12, resulting in an increase of thrust of the pusher 15 and pushing force between the trays 2. Therefore, when number of trays 2 used is increased, then upper faces of the skid beams 12 constituting a transportation path of the trays 2 may be deformed in a wave shape or formed with steps, with the disadvantageous result that the column of trays 2 on the skid beams 12 are not smoothly slid and may lift up like a bridge as shown in FIG. 3 leading to failure of transportation of the trays.
If push-in load for the column of trays 2 applied by the pusher 15 is increased in this state, then the trays 2 may jump upwardly and buckle.
Furthermore, the amount of input heat conducted to the material 1 via the trays 2 from below is inevitably less than that conducted from above or from each side since, with the trays 2 being supported by the skid beams 12 longitudinally running through the furnace body 5, the material 1 is heated by the heaters 14 at opposite sides of the path of transportation of the trays 2 so that heating of the material 1 may be insufficient at its lower portion, thereby decreasing production yields.
Heat treatment time period for ceramics are generally predetermined. Therefore, in order to increase the amount of production, the length of the furnace must be prolonged and transportation speed (tact) of the trays must be increased, which will thus cause an increase in the number of trays 2 in the furnace. The skid-type transportation mechanism is low in transportation limit, resulting in restriction in number of trays 2 in the furnace.
Generally speaking, installation and running costs are lowered as the amount of production is increased. The transportation limit may be a restriction with respect to cost.
An increase in friction force will cause an increase in horizontal force generated in the furnace floor, resulting in the necessity of an increase in size of the furnace floor structure. Overcoming this problem by changing the furnace material is difficult to attain since there is no effective material other than graphite as to a high-temperature furnace. An increase in size of the furnace floor structure will eventually result in the deterioration of uniformity of heat above and below the material to be sintered, leading to poor yields. In other words, the area of the effective zone for sintering is decreased in the furnace, thereby lowering the amount of production. This means a decrease in heating efficiency of the furnace and an increase in installation and running costs.
An increase in friction force will also accelerate wear between the tray 2 and the skids 12. This causes a deviation of the levels of the skids 12 and the levels of the trays 2, leading to lift-up of a bridge. As a result, transportation limit of the trays may be generated due to aging, which may make it difficult to effect stable operation.
The heaters 14 extend vertically and are arranged laterally of the trays. In a high-temperature furnace, the heaters 14 may reach temperatures of more than 2,000xc2x0 C. so that the electrodes used need to be water-cooled. In order to absorb heat expansion of the heaters 14 themselves (e.g., 10 mm or more for 1 m of heater), the heaters 14 are fixed at a top portion thereof and are free at their lower ends. In order to attain uniformity of heat, heat loss at the furnace floor must be compensated. However, in the case of such lateral arrangement of the heaters 14, vertical heat input cannot be controlled. To this end, the furnace height may be increased over and above what is needed, prolonging the heater length. However, then vertical heat input is fixed. Moreover, to increase in size of the furnace over and above what is needed may result in decrease of heating efficiency and increase in installation and running costs.
In view of the foregoing, the invention has as one of its objects to provide a continuous sintering furnace and use thereof which can enhance energy efficiency of the furnace as a whole and ensure transportation of trays.
In order to attain the above-mentioned object, a continuous sintering furnace according to the present invention comprises an entrance-side deaerating chamber through which trays each with a material to be sintered being mounted thereon may pass, preheating, heating and cooling zones into which the trays are sequentially fed from said entrance-side deaerating chamber, an exit-side deaerating chamber through which the trays having passed through the cooling zone may pass, a pusher for pushing the tray from the entrance-side deaerating chamber to the preheating zone, a puller for pulling the tray from the cooling zone to the exit-side deaerating chamber, an intermediate puller for pulling the tray from the heating zone to the cooling zone, a vertically movable first door between the entrance-side deaerating chamber and the preheating zone, a vertically movable first intermediate door adjacent to said first door and arranged at an upstream end of the preheating zone in the direction of transportation of the trays, a vertically movable second intermediate door between the heating and cooling zones and a vertically movable second door between the cooling zone and the exit-side deaerating chamber.
In a continuous sintering furnace according to the claimed invention, a number of free rollers for supporting the trays from below are arranged over whole lengths of the preheating, heating and cooling zones.
A continuous sintering furnace according to the present invention comprises a substantially horizontally arranged furnace body through which a plurality of trays each with a material to be sintered being mounted thereon may pass from one end to the other end of the furnace body, a number of free rollers for supporting the trays from below which are arranged in a spaced apart relationship over whole length of said furnace body, a plurality of lower heaters arranged over a predetermined range in the furnace body so as to be positioned below and between the free rollers and a plurality of upper heaters arranged over the predetermined range in the furnace body so as to be positioned above a path of transportation of the trays.
In a continuous sintering furnace according to the invention, in addition to the constructions of the continuous sintering furnace according to the claimed invention, the free rollers are arranged in a plurality of columns along the length of the furnace body.
In a continuous sintering furnace according to the present invention, in addition to the constructions of the continuous sintering furnace according to the claimed invention, the lower and upper heaters extend horizontally and laterally of the trays and are arranged symmetrically with respect to the path of transportation of the trays.
In a continuous sintering furnace according to the claimed invention, in addition to the constructions of the continuous sintering furnace according to the present invention, each of the lower and upper heaters comprises a heating energization body which extends through side walls of the furnace body substantially horizontally and laterally of the trays, and holders which support electrodes on opposite ends of the heating energization body.
In a continuous sintering furnace further according to the claimed invention, when the continuous sintering furnace is used, a push-in load of the pusher at which the column of trays pushed downstream in the direction of transportation lift up in a bridge shape is preliminarily grasped as a push interrupt load wherein if the push-in load of the pusher reaches said push interrupt load upon pushing of the trays downstream in the direction of transportation, the operation of the pusher is temporarily interrupted and the lift-up of the column of trays is eliminated, and then the trays 22 are pushed again.
In the continuous sintering furnaces also according to the present invention, the trays are pulled one by one from the heating zone to the cooling zone by the intermediate puller; and, with the second intermediate door being at its lowered position for closing, heat input to the cooling chamber is suppressed.
In the continuous sintering furnace further according to the claimed invention, a number of free rollers disposed in the preheating, heating and cooling zones support the trays, thereby facilitating the transportation of the trays.
In the continuous sintering furnaces claimed, radiant heat energy from the upper heaters is conducted to the material to be sintered from above, and radiant heat energy from the lower heaters is conducted to the material to be sintered from below through clearances between the free rollers and via the trays so that the material to be sintered are heated.
The arrangement of the heaters so as to be above and below the material to be sintered can vary input heat vertically. The furnace floor receives a load of the material to be sintered and in contact with the furnace body which is at low temperature so that inevitably thermal loss will generate and the work or material has temperature distribution or deviation, which will be compensated by heat input of the horizontal heaters arranged above and blow the material to be heated.
In a high-temperature furnace, the horizontal heaters must absorb thermal expansion of the heaters themselves and thermal expansion of the heating zone in the directions longitudinally and laterally of the furnace. Actually, the furnace body has weld structures of iron and therefore has manufacturing tolerances. These problems are solved by a one-point support mechanism using O-rings at opposite ends of the heater (mechanism which supports each end of the heater at a point and which receives thermal expansion of the heater). This can absorb the thermal expansions of the heater and heating zone and the manufacturing tolerances of the furnace body.
In the continuous sintering furnace according to claim 4, the trays are supported by the free rollers arranged in a plurality of columns along the length of the furnace body, thereby improving the conductive efficiency of radiant heat energy from the lower heaters to the materials to be sintered.
In comparison with the skid beam system, the free roller system reduces the friction between tray and free rollers so as to be about one tenth as much as that of the skid beam system. As a result, the following advantages are obtained:
1. A decrease in friction force will cause a decrease in thrust of the pusher, leading to a decrease in internal force between the adjacent trays, which will improve the transportation limit and increase the number of trays transportable in the furnace and the amount of production. Experiments revealed that lift-up of a bridge of trays is a function of the horizontal level of trays (level of furnace floor) and internal force between the adjacent trays and that the internal force between the adjacent trays is substantially in proportion to transportation limit. Therefore, the transportation limit is increased by substantially ten times.
2. The frictional force provides a horizontal force in the furnace floor so that decrease in friction force will make it possible to decrease in size of furnace floor structure. In a high-temperature furnace, which has restriction in selection of material, such decrease in friction force is extremely effective means for making the furnace floor smaller-sized. Because of the furnace floor being smaller-sized, the effective space in the furnace is increased. In a high-temperature furnace, in which radiation of the heaters is dominant in heating of the material to be sintered, such increase in space results in an increase in heating efficiency. As to thermal loss of the furnace floor, which is dependent upon area thereof in the furnace body, a decrease in horizontal force leads to decrease in number of furnace floor support pillars, whereby the thermal loss can be decreased.
Thermal uniformity of the work or material is dependent upon balance of thermal loss. Decrease in thermal loss on the furnace floor is effective to this, leading to improvement of thermal uniformity. This will increase the effective zone, leading to improvement of production yields, which in turn results in decrease in installation and running costs.
3. Because of the furnace floor being smaller-sized, the effective space is increased, which enables an arrangement of effective horizontal heaters. The heater structure sandwiching the material to be sintered from above and below can compensate thermal loss of the furnace floor.
4. The decrease in friction force leads to decrease in wear between the trays and rollers. This will decrease variation of the furnace floor level due to wear. As a result, lift-up of a bridge is suppressed, the operation of the furnace is stabilized and maintenance intervals are prolonged.
In the continuous sintering furnace according to claim 5, the lower and upper heaters are arranged substantially horizontally and laterally of the trays and symmetrically with respect to the path of transportation of the trays, thereby making temperature distribution in the materials to be heated in its lateral direction uniform.
In the continuous sintering furnace according to the claimed invention, the heating energization body extends through the side walls of the furnace body, and the electrodes at the opposite ends of the energization body are displaceably supported by the holders, so that the difference in thermal expansion between the heating energization body and the furnace body is absorbed.
In use of a continuous sintering furnace according to the claimed invention, heat input to the cooling chamber is suppressed such that the trays are moved from the heating zone to the cooling chamber by the intermediate puller and the intermediate door can be closed into its lowered position.
In use of a continuous sintering furnace according to the present invention, whether or not the column of trays lift up is judged based on whether or not the push-in load of the pusher has reached the push interrupt load preliminarily grasped. If the push-in load of the pusher has reached the push interrupt load, the operation of the pusher is temporary interrupted and the lift-up of the column of trays is eliminated. Conditions of the column of trays where the lift-up occurred may be changed to advance the column of the trays depending upon the push-in load of the pusher.